Method of reclaiming contaminated metal

ABSTRACT

A method of reclaiming a metal M 1  from a source of highly contaminated metal M 1 X having a high oxygen content includes the steps of: sintering the highly contaminated finely divided metal M 1 X into a perform; introducing the preform into an electrochemical cell, the cell containing a liquid electrolyte comprising a fused salt or mixture of salts generally designated as M 2 Y in which oxygen is soluble, and a relatively inert anode; conducting electrolysis under conditions favorable to the selective dissolution of oxygen in preference to the deposition of the M 2  cation to form a decontaminated metal preform; and following electrolysis, reclaiming the decontaminated metal preform from the cathode. The reclaimed decontaminated metal preform may optionally be crushed to yield a powder of the metal M 1 . The highly contaminated finely divided metal may comprise swarf, chips and such-like from machining operations, or may comprise high oxygen content metal powders such as hydride-dehydride (HDH) titanium powder. By using the method of the present invention, the oxygen content of the finely divided metal can be reduced to less than 4000 ppm, preferably less than 2000 ppm, more preferably less than 1000 ppm and even more preferably less than 500 ppm.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of application Ser. No. 10/204,465 filed Sep. 10, 2002 which in turn is a §371 of PCT/GB01/00653 filed Feb. 19, 2001.

FIELD OF THE INVENTION

This invention relates to a method for reclaiming a metal from a highly contaminated material. More particularly, the invention is directed to reclaiming finely divided metals contaminated with a high level of oxygen, or producing alloys therefrom. Such metals and alloys preferably comprise titanium.

BACKGROUND OF THE INVENTION

Okabe et al have described the removal of oxygen from small amounts of titanium wires and titanium pieces using an electrochemical technique (Metallurgical Transactions B, Vol. 24B, June 1993, 449-455).

WO 99/64638 to Fray et al and the Applicant's co-pending applications WO 01/62995 and WO 01/62996 (the disclosures of which are incorporated herein by reference) describe methods for the electrolytic reduction of metal compounds, particularly metal oxides.

Certain embodiments of these methods involve the electrolysis of metal oxides or other compounds (M₁X) in a cell containing a liquid (fused salt M₂Y) electrolyte and an anode, the metal oxide or other compound forming the cathode. Conditions are controlled so as to bring about the selective dissolution of the oxygen or other contaminant of the cathode in preference to deposition of the metal cation. Improved efficiency of this process can be achieved by various methods as described in WO 01/62995 and WO 01/62996 some of which are summarised below. M₁X may also be a metal M₁ contaminated by an element X, for example a metal contaminated with a high level of oxygen.

When the element X is oxygen, the electrolytic reduction process is commonly referred to as electro-deoxidation (EDO).

BRIEF DESCRIPTION OF THE INVENTION

The present invention provides a method of reclaiming a metal M₁ from a source of highly contaminated metal M₁X having a high oxygen content, the method including the steps of: sintering the highly contaminated finely divided metal M₁X into a preform; introducing the preform into an electrochemical cell, the cell containing a liquid electrolyte comprising a fused salt or mixture of salts generally designated as M₂Y in which oxygen is soluble, and a relatively inert anode; conducting electrolysis under conditions favorable to the selective dissolution of oxygen in preference to the deposition of the M₂ cation to form a decontaminated metal preform; and following electrolysis, reclaiming the decontaminated metal preform from the cathode.

The reclaimed decontaminated metal preform may optionally be crushed to yield a powder of the metal M₁.

The source of highly contaminated metal may optionally be mixed with oxides of M₁ or oxides of alloying elements in the form of refined oxide or raw ores.

By using the method of the present invention, the oxygen content of the finely divided metal can be reduced to less than 4000 ppm, preferably less than 2000 ppm, more preferably less than 1000 ppm and even more preferably less than 500 ppm.

Manufacturing operations generate large quantities of waste material, often as swarf and chips, some of which can be processed and recycled. However, certain operations such as cutting create high temperatures in the swarf and chips and cause absorption of high quantities of oxygen into these pieces. The high surface area to volume ratio of such pieces may result in penetration of oxygen through almost the entire body of the piece. Beyond an optimum, increasing quantities of oxygen in the metal have an increasingly detrimental effect on its mechanical properties and the metal cannot be used directly. Typically, titanium machining swarf from forgings and such-like contains between 3000 and 4000 ppm oxygen.

Where the quantities of oxygen absorbed are low to moderate, it is known to mix these waste products with quantities of scrap having lower levels of oxygenation and/or with quantities of virgin metals to ensure that the end product of the mixture has below a certain desired maximum oxygen content. At higher levels of contamination, it becomes increasingly uneconomical to dilute the highly contaminated metal in this manner and consequently, much of this potentially useful material is abandoned as redundant. However, such material can be readily recovered by the process of the present invention.

Swarf, chips and such-like from machining operations are not the only example of a finely divided metal having a high content of oxygen which can be used as raw material for the process of the current invention. High oxygen content metal powders are commercially available, one example being hydride-dehydride (HDH) titanium powder. HDH titanium is made by embrittling titanium sponge, titanium ingot or titanium scrap in hydrogen, crushing the embrittled Ti to powder and then dehydriding the material back to its original state. The resulting powder is generally cheap to make, but has high impurity levels, especially of oxygen. The oxygen level in HDH titanium is typically 5,000-6,000 ppm, which level precludes the use of HDH powder for high performance applications such as aerospace and automotive parts. Another disadvantage of HDH titanium powder is that the particles are angular in nature, whereas metal powders formed by alternative techniques such as gas atomisation are spherical.

The component pieces or particles of the finely divided metal may have either an irregular shape or a uniform shape. However, a particular advantage of the present invention is realized when the finely divided metal is a powder comprising irregularly-shaped particles, such as in the case of HDH powder. It has unexpectedly been found that a sintering step in combination with the electro-deoxidation process converts the angular HDH particles to less angular or even near-spherical particles, thereby significantly enhancing the flowability of the decontaminated product after the optional crushing step. Titanium powders produced in this way are suitable for use in the powder metallurgy industry.

Thus, by using cheap, low quality HDH powder as a feedstock for the electrolytic reduction process, high quality titanium powder can be obtained at an economical cost. Moreover, by adding metal oxides at levels equivalent to typical alloying additions for titanium to the HDH powder, a method of manufacturing titanium alloy powders is also obtained. Alternatively, finely divided alloying metals, with or without a high level of oxygen contamination, may be added to the initial HDH powder to obtain titanium alloy.

Optionally, the electrolysis is performed on a preformed sintered mass comprising a mixture of highly contaminated metal made up of a proportion of particles of size generally greater than 20 microns and a proportion of finer particles of less than 7 microns. Preferably the finer particles make up between 10 and 55% by weight of the sintered block.

High density granules of approximately the size required for the powder are manufactured and then are mixed with very fine unsintered metal or metal oxide (e.g., titanium dioxide), binder and water in the appropriate ratios and formed into the required shape of feedstock. This feedstock is then sintered to produce a preform having the required strength for the reduction process. The resulting feedstock, after sintering but before reduction, consists of high density granules in a lower density (porous) matrix.

The preform can be reduced in block form using the previously described electrolytic method and the result is a friable block which can easily be broken up into powder.

The finely divided metal may comprise pieces or particles ranging from a few microns to several mm in size, and may be a powder. In one preferred embodiment, the finely divided metal is formed by crushing machining chips, swarf or such-like to pieces of size 2-5 mm. In another preferred embodiment, the finely divided metal is HDH titanium powder, typically having a particle size of between 45 and 250 μm.

One convenient method of subjecting a finely divided metal to electrolysis would be to place the metal pieces or powder in a basket or mesh electrode. However, the present inventors have found that, contrary to expectations based on work with metal oxides and the small scale experiments of Okabe et al, the electrolytic reduction process does not proceed reliably when a basket electrode is used. Indeed, for larger quantities of metal, and for very finely divided metal or powders, the electro-deoxidation reaction did not proceed at all. Unexpectedly, it has been found that the reliability and effectiveness of the electrolytic reduction process is significantly improved by sintering the finely divided metal together, either under vacuum or in air, to produce a preform with sufficient mechanical properties to withstand the electrolytic reduction process. It is not entirely clear why this is the case, but one possibility is that surface tension effects in the molten electrolyte inhibit electrical contact between unsintered metal fragments and particles. By sintering the particles or fragments together, good electrical contact is maintained. However, the scope of the invention is not limited by this explanation

Thus, in order to obtain a workable scaled up process for the recovery of scrap metal by EDO, or the purification of metal powders by EDO, it is regarded as essential that the metal is first sintered together. The present invention is of especial application in scaled up processes for the recovery of scrap metal where more than 10 g, preferably more than 50 g, and usually more than 100 g of swarf, chips or scrap undergoes electro-deoxidation in a single cell.

Ideally, the sintering is sufficient to obtain a degree of diffusion bonding between the metal pieces or particles, but still maintain an open porosity in the electrode. In other words, there is electrical contact between the fragments or particles, but the final density is less than 90-92% of the theoretical fully compacted density. In a preferred embodiment, the sintering step imparts sufficient strength to the preform for it to act as a cathode, but the sintering is still light enough to produce an electrolytically reduced product which is friable. By lightly sintering powdered metal in this way, good electrical contact between particles can be achieved while still obtaining a decontaminated product which can be easily crushed into a powder. Optionally, the finely divided metal is cold pressed prior to sintering so as to shape the preform. Sintering from a dry mixture tends to produce a preferable lower density than sintering from a cast slurry.

In order to obtain a preform which can be crushed following the electrolytic reduction step, the finely divided metal is sintered such that the density of the preform increases by no more than 5%, preferably by no more than 2% and more preferably by between 1% and 2%. That result can be achieved by using short sintering times, low sintering temperatures or a combination thereof. For titanium, the sintering step typically takes less than 4 hours, preferably less than 3 hours and even more preferably between 1 and 2 hours, at a temperature of between 850 and 1300° C.

In any of the methods as described by Fray et al, X may be a metalloid such as oxygen, sulfur, carbon or nitrogen. Although the method is applied to materials with high oxygen content, other metalloids may be removed at the same time. M₁ may be a Group IV element such as Ti, Si, Ge, Zr or Hf, a lanthanide such as Sm or Nd, a transition metal such as Mo, Cr, Nb or Sc, or an alloy of any of the preceding metals. Preferably, M₁ comprises titanium. A preferred electrolyte, M₂Y, is calcium chloride (CaCl₂). Other suitable electrolytes include but are not limited to the molten chlorides of all common alkali and alkaline earth metals, or mixtures thereof. Other preferred metals for M₂ are barium, caesium, lithium, strontium and yttrium. The anode of the cell is preferably formed from a material which is relatively inert and insoluble. One suitable anode material is graphite.

Processing conditions suitable for the favorable dissolution of oxygen require that the potential of the cell preferably be maintained at a potential which is less than the decomposition potential of the molten electrolyte M₂Y during the process. Allowing for polarisation and resistive losses in the cell, it will be understood that the cell potential may be maintained at a level equal to, or marginally higher than, the decomposition potential of M₂Y and still achieve the desired result. Potentiostatic methods may be used to control the potential.

It is also preferred that the temperature of the cell is maintained at an elevated temperature which is significantly above the melting point of M₂Y but below the boiling point of M₂Y. Where M₂Y is CaCl₂, suitable processing parameters include a potential of up to about 3.3V and a processing temperature of between about 825 and 975° C.

The method may include an additional step wherein the scrap metal may be processed before being introduced into the electrochemical cell, for example to form small granules, or a powder, or an amorphous slurry of the contaminated material.

Alternatively, or in addition to the additional processing step, the scrap material (and metal oxide if used) may be sintered in a bimodal mixture containing particles greater than 20 microns in size and finer particles less than about 7 microns in size, binder and water to form a friable block. Preferably in this method, the finer particles are in a proportion of about 10 to about 55% by weight of the sintered block.

Alternatively or in addition to the additional processing step, the scrap metal may be fabricated into a ceramic facsimile of a desired metal or metal alloy component before introduction into the electrochemical cell. Optionally, further oxides of the M₁ or alloying elements may be included in the facsimile. This fabrication may be achieved by various known methods including pressing, injection molding, extrusion and slip casting followed by sintering. Full density of the metallic component can be achieved by sintering with or without the application of pressure, either in the electrochemical cell, or in a subsequent operation. Shrinkage of the component during the conversion to metal or alloy should be allowed for by making the ceramic facsimile proportionally larger than the desired component.

EXAMPLES

The following Examples illustrate the invention:

Comparative Example 1

500 g titanium swarf having an initial oxygen level of 3200 ppm was placed in a stainless steel basket which was made the cathode of an electrolytic cell containing CaCl₂ electrolyte. Electrolysis was carried out under standard conditions (950° C., 24 hours and 3V), but it was found that the oxygen level had increased at the end of the run to 9000 ppm.

Comparative Example 2

This was a repeat of Comparative Example 1. At the end of the run, the oxygen level in the titanium swarf had increased to 7900 ppm.

Comparative Example 3

500 g titanium swarf having an initial oxygen level of 3200 ppm was placed in a stainless steel basket which was made the cathode of an electrolytic cell containing a 30:70 wt % NaCl:CaCl₂ eutectic mix. Electrolysis was carried out under standard conditions (950° C., 24 hours and 3V), but it was found that the oxygen level had increased at the end of the run to 8600 ppm.

Example 4

200 g titanium chips having an initial oxygen level of 3200 ppm were cold pressed into thick discs and vacuum sintered at 1150° C. for 1 hour. The discs were placed in a stainless steel basket which was made the cathode of an electrolytic cell containing CaCl₂ electrolyte. Electrolysis was carried out under standard conditions (950° C., 24 hours and 3V) and at the end of the run the oxygen level had decreased to 2600 ppm.

Example 5

194 g titanium chips having an initial oxygen level of 3200 ppm were cold pressed into thick discs having a central hole and vacuum sintered at 1150° C. for 1 hour. The discs were threaded onto a stainless steel cathode and subjected to electro-deoxidation in a CaCl₂ electrolyte for 24 hours at 950° and 3V. At the end of the run, the oxygen content of the titanium had been reduced to 223 ppm.

Example 6

This was a repeat of Example 5. At the end of the electrolysis run, the oxygen content of the titanium discs had reduced to 518 ppm.

Example 7

162 g titanium chips having an initial oxygen level of 3200 ppm were cold pressed into thick discs and vacuum sintered at 1150° C. for 1 hour. The discs were threaded onto a stainless steel cathode and subjected to electro-deoxidation in a 30:70 wt % NaCl:CaCl₂ eutectic mix for 24 hours at 950° and 3V. At the end of the run, the oxygen content of the titanium had been reduced to 1328 ppm.

Examples 8 and 9

The reactive metals Hf and Sc were identified as candidates for the electrolytic removal of dissolved oxygen. Samples of both of these metals were obtained in the form of finely divided scrap which was high in oxygen (several percent). The chips were put into a stainless steel basket which was made the cathode in the electrolytic reduction process. The mesh size of the basket was as large as possible given the size of the chips (about 1 mm hole size). Electrolysis was carried out under standard conditions (950° C., 24 hours and 3V), but it was found that the oxygen level could not, in either case, be reduced during electrolysis. This demonstrates that the problem identified with the reduction of Ti chips is generic to other metals.

Example 10

HDH titanium powder was poured into a small ceramic crucible and vacuum sintered to a porous block at 1200° C. for 2 hours, and subsequently electrolysed at 950° C. and 3V for 24 hours, using a CaCl₂ electrolyte. The initial oxygen concentration of the powder was about 4000 ppm, but after electrolysis the oxygen level was below 2000 ppm. The sintered HDH powder had sufficient strength to be subjected to the electrolysis process, but the resulting decontaminated product could still be re-crushed to powder. 

1. A method of reclaiming a metal M₁ from a source of highly contaminated metal M₁X having a high oxygen content, said method including the steps of: sintering the highly contaminated finely divided metal M₁X into a preform; introducing the preform into an electrochemical cell, the cell containing a liquid electrolyte comprising a fused salt or mixture of salts generally designated as M₂Y in which oxygen is soluble, and a relatively inert anode; conducting electrolysis under conditions favorable to the selective dissolution of oxygen in preference to the deposition of the M₂ cation to form a decontaminated metal preform; and following electrolysis, reclaiming the decontaminated metal preform from the cathode.
 2. A method according to claim 1, wherein the sintering step increases the density of the preform by no more than 5%.
 3. A method according to claim 2, wherein the sintering step increases the density of the preform by no more than 2%.
 4. A method according to claim 1, wherein M₁ comprises titanium and the finely divided metal is sintered for less than 4 hours.
 5. A method according to claim 1, wherein the finely divided metal is formed by crushing chips or swarf.
 6. A method according to claim 5, wherein the finely divided metal has a size of 2 mm to 5 mm.
 7. A method according to claim 1, wherein the highly contaminated finely divided metal is hydrogen-dehydride (HDH) titanium powder.
 8. A method according to claim 6, wherein the HDH powder has a particle size of between 45 μm and 250 μm.
 9. A method according to claim 6, wherein a metal oxide is mixed with the HDH powder in an amount equivalent to an alloying addition.
 10. A method according to claim 1, wherein M₁ is selected from the group consisting of a Group IV element, Ti, Si, Ge, Zr or Hf, a lanthanide Sm or Nd, a transition metal Mo, Cr, Nb or Sc, and an alloy comprising any combination thereof.
 11. The method according to claim 10 wherein M₁ is Ti, Si, Ge, Zr or Hf.
 12. The method according to claim 10 wherein M₁ is Sm or Nd.
 13. The method according to claim 10 wherein M₁ is Mo, Cr, Nb or Sc.
 14. A method according to claim 1, wherein M₁ is titanium or a titanium alloy.
 15. A method according to claim 1, wherein the preform also contains metal oxide.
 16. A method according to claim 1, wherein M₂Y is CaCl₂. 